In evolutionary biology and evolutionary psychology, the Trivers–Willard hypothesis, formally proposed by Robert Trivers and Dan Willard, suggests that female mammals are able to adjust offspring sex ratio in response to their maternal condition. For example, it may predict greater parental investment in males by parents in "good conditions" and greater investment in females by parents in "poor conditions" (relative to parents in good condition). The reasoning for this prediction is as follows: Assume that parents have information on the sex of their offspring and can influence their survival differentially. While pressures exist to maintain sex ratios at 50%, evolution will favor local deviations from this if one sex has a likely greater reproductive payoff than is usual.
Trivers and Willard also identified a circumstance in which reproducing individuals might experience deviations from expected offspring reproductive value—namely, varying maternal condition. In polygynous species males may mate with multiple females and low-condition males will achieve fewer or no matings. Parents in relatively good condition would then be under selection for mutations causing production and investment in sons (rather than daughters), because of the increased chance of mating experienced by these good-condition sons. Mating with multiple females conveys a large reproductive benefit, whereas daughters could translate their condition into only smaller benefits. An opposite prediction holds for poor-condition parents—selection will favor production and investment in daughters, so long as daughters are likely to be mated, while sons in poor condition are likely to be out-competed by other males and end up with zero mates (i.e., those sons will be a reproductive dead end).
The hypothesis was used to explain why, for example, Red Deer mothers would produce more sons when they are in good condition, and more daughters when in poor condition. In polyandrous species where some females mate with multiple males (and others get no matings) and males mate with one/few females (i.e., "sex-role reversed" species), these predictions from the Trivers–Willard hypothesis are reversed: parents in good condition will invest in daughters in order to have a daughter that can out-compete other females to attract multiple males, whereas parents in poor condition will avoid investing in daughters who are likely to get out-competed and will instead invest in sons in order to gain at least some grandchildren.
"Condition" can be assessed in multiple ways, including body size, parasite loads, or dominance, which has also been shown in macaques (Macaca sylvanus) to affect the sex of offspring, with dominant females giving birth to more sons and non-dominant females giving birth to more daughters. Consequently, high-ranking females give birth to a higher proportion of males than those who are low-ranking.
The Trivers–Willard hypothesis rests on certain assumptions:
- Parental condition is associated with offspring condition;
- Differences in offspring condition will persist into adulthood;
- Being in condition differentially affects the mating success of one sex more than it does the other.
Evolutionary biologists predict a Trivers–Willard effect where these conditions hold, and no effect when these conditions do not hold. In polygynous species where some males have multiple mates and others have none (i.e., greater variance in mating success among males than females), being in good condition affects males more than females. This is reversed in polyandrous species, and possibly in species where condition is based on social status and males disperse.
In their original paper, Trivers and Willard were not yet aware of the biochemical mechanism for the occurrence of biased sex ratios. Eventually, however, Melissa Larson et al. (2001) proposed that a high level of circulating glucose in the mother's bloodstream may favor the survival of male blastocysts. This conclusion is based on the observed male-skewed survival rates (to expanded blastocyst stages) when bovine blastocysts were exposed to heightened levels of glucose. As blood glucose levels are highly correlated with access to high-quality food, blood glucose level may serve as a proxy for "maternal condition". Thus, heightened glucose functions as one possible biochemical mechanism for observed Trivers–Willard effects.
The Trivers–Willard hypothesis has been applied to resource differences among individuals in a society as well as to resource differences among societies. Empirical evidence is mixed with higher support in better studies according to Cronk in a 2007 review. One example, in a 1997 study, of a group with a preference for females was Romani in Hungary, a low-status group. They "had a female-biased sex ratio at birth, were more likely to abort a child after having had one or more daughters, nursed their daughters longer, and sent their daughters to school for longer".
Following up on a review of empirical studies that shows increasing support for the Trivers–Willard hypothesis for status measurement that are closer around the time of conception, a study on U.S. billionaires finds a similar timing effect for humans. The author draws on a sample of the Forbes 400 in 2009 and shows that the effect can be found among male billionaires that inherited their wealth but not among those who are self-made billionaires. A possible reason for this is that heirs as opposed to self-made billionaires were rich before they had any of their children. Although conditionally consistent with the Trivers–Willard hypothesis for men, but not for women, the study fails to replicate earlier results on the purported strength of the effect. A reason for the inconsistency of the results is that previous research was based on data with a higher proportion of missing data and therefore subject to sample selection.
- Trivers, R. L.; Willard, D. E. (1973). "Natural selection of parental ability to vary the sex ratio of offspring". Science 179 (4068): 90–92. doi:10.1126/science.179.4068.90. PMID 4682135.
- Kuesterl, A. Paul; et al. (1992). "Maternal rank affects reproductive success of male Barbary macaques (Macaca sylvanus): evidence from DNA fingerprinting". Behavioral Ecology and Sociobiology 30 (5): 337–341. doi:10.1007/BF00170600.
- Larson, M.; et al. (2001). "Sexual Dimorphism among Bovine Embryos in Their Ability to Make the Transition to Expanded Blastocyst and in the Expression of the Signaling Molecule IFN-τ". Proc. Natl. Acad. Sci. USA 98 (17): 9677–9682. doi:10.1073/pnas.171305398.
- Lieberman, Leslie (2003). "Dietary, Evolutionary and Modernizing Influences on the Prevalence of Type 2 Diabetes". Annual Review of Nutrition 23: 345–377. doi:10.1146/annurev.nutr.23.011702.073212.
- Cronk, L. (2007). "Boy or girl: Gender preferences from a Darwinian point of view". Reproductive BioMedicine Online 15: 23–32. doi:10.1016/S1472-6483(10)60546-9. PMID 18088517.
- Cameron, E. Z. (2004). "Facultative adjustment of mammalian sex ratios in support of the Trivers-Willard hypothesis: Evidence for a mechanism". Proceedings of the Royal Society B: Biological Sciences 271 (1549): 1723–1710. doi:10.1098/rspb.2004.2773.
- Schnettler, S. (2013). Sorci, Gabriele, ed. "Revisiting a Sample of U.S. Billionaires: How Sample Selection and Timing of Maternal Condition Influence Findings on the Trivers-Willard Effect". PLoS ONE 8 (2): e57446. doi:10.1371/journal.pone.0057446. PMID 23437389.
- Cameron, E. Z.; Dalerum, F. (2009). Reby, David, ed. "A Trivers-Willard Effect in Contemporary Humans: Male-Biased Sex Ratios among Billionaires". PLoS ONE 4 (1): e4195. doi:10.1371/journal.pone.0004195. PMC 2614476. PMID 19142225.